Method for optimizing acoustic source array performance

ABSTRACT

A technique facilitates obtaining seismic data in a marine environment. An array of acoustic sources is deployed in a marine environment. The array can be utilized for creating acoustic pulses that facilitate the collection of data on subsea structures. The methodology enables optimization of acoustic source array performance to improve the collection of useful data during a seismic survey.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/337118 filed Dec. 17, 2008, which is incorporated herein byreference in its entirety.

BACKGROUND

In a variety of marine environments, seismic surveys are conducted togain a better understanding of geological formations beneath a body ofwater. Marine seismic source arrays are used to generate acoustic pulsesin the water, and hydrophones detect the reflected signals. Firingcontrollers are employed to trigger the firing of the acoustic sourceelements so the main pressure pulse of each element is synchronized inthe farfield in the desired direction. For example, the triggering maybe conducted such that the primary pulses of each element coincide in avertical direction in the farfield. In some applications, the firingcontroller implements time delays to compensate for individualvariations in the mechanical triggering mechanisms of the acousticsource elements. Triggering delays can also be used to compensate forgeometric variations of the source array.

One approach to quantifying the mechanical triggering delay is to use atime-break sensor. The time-break sensor is positioned inside orproximate the acoustic source element, e.g., air gun, and a specificattribute is detected in the signal measured by the time-break sensor.For example, the attribute may comprise signal maximum amplitude, timeof threshold, zero-crossing, or other suitable attributes. The timedelay between sending the firing signal and the time of the detectedattribute in the time-break signal is processed via a firing controlalgorithm to adjust the time of the next firing signal.

However, acoustic source element synchronization using air gun mountedtime-break sensors provides only an indirect way of synchronizing thepeak pressure of the emitted acoustic signals. The approach assumes aconstant, source element independent, time offset between the detectedattribute in the time-break signal and the time of peak acousticpressure. In many applications, this assumption is not valid and thetime-break synchronization results in sub-optimal alignment of peakacoustic pressure of the acoustic signals. Sometimes, the problem may bemitigated by using firing controllers that support tuning andmeasurements from nearfield hydrophones. However, with modern compactarray configurations it is not possible to distinguish acoustic signalsfrom adjacent air guns in the unprocessed nearfield hydrophonemeasurements.

Another problem with conventional air guns is the emission ofsignificant acoustic amplitude outside of the frequency range ofinterest for seismic exploration. The out of band signal representsnoise that can interfere with measurements and/or have an adverse affecton marine life.

SUMMARY

In general, the present invention provides a methodology for obtainingseismic data in a marine environment. An array of acoustic sources isdeployed in a marine environment. The array can be employed in creatingacoustic pulses which are useful in obtaining data on subsea structures.The methodology enables optimization of acoustic source arrayperformance to facilitate the collection of useful data during a seismicsurvey.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the invention will hereafter be described withreference to the accompanying drawings, wherein like reference numeralsdenote like elements, and:

FIG. 1 is a schematic view of an acoustic source array in a marinesurvey area, according to an embodiment of the present invention;

FIG. 2 is a schematic view of another example of an acoustic sourcearray, according to an embodiment of the present invention;

FIG. 3 is a diagram showing locations of acoustic source/hydrophonepairs in one example of an acoustic source array, according to anembodiment of the present invention;

FIG. 4 is a diagram illustrating data from a signal detected at one ofthe hydrophones based on contributions from the acoustic source elementsin an array such as the array illustrated in FIG. 3, according to anembodiment of the present invention;

FIG. 5 is a diagram illustrating a power spectrum of a scatteringfunction, according to an embodiment of the present invention;

FIG. 6 is a diagram illustrating another power spectrum of a scatteringfunction, according to an alternate embodiment of the present invention;

FIG. 7 is a diagram illustrating another power spectrum of a scatteringfunction, according to an alternate embodiment of the present invention;

FIG. 8 is a diagram illustrating a power spectrum arising fromdeliberate scattering of the firing times within a given window of time,according to an embodiment of the present invention;

FIG. 9 is a flowchart illustrating one example of an operationalprocedure for optimizing source array performance, according to anembodiment of the present invention; and

FIG. 10 is a flowchart illustrating another example of an operationalprocedure for optimizing source array performance, according to anembodiment of the present invention.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those of ordinary skill in the art that the presentinvention may be practiced without these details and that numerousvariations or modifications from the described embodiments may bepossible.

The present invention generally relates to a technique for use inobtaining seismic data in a marine environment. The technique aids inthe optimization of acoustic source array performance with respect toacoustic source arrays employed in conducting seismic surveys. In oneapplication, for example, the technique facilitates the attenuation ofhigh frequency output from acoustic sources by desynchronizing thesource triggering. In an alternate aspect of the technique, the acousticsources may be synchronized using band-limited notional sourcesignatures.

Referring generally to FIG. 1, an example of a seismic survey system 20is illustrated according to one embodiment of the present invention. Asillustrated, system 20 comprises an acoustic source subarray 22 that istowed through a marine seismic survey area 24 to conduct a marineseismic survey. The acoustic source subarray 22 may be towed by asuitable surface vessel connected to acoustic source subarray 22 via atow line.

In the example illustrated, seismic survey system 20 further comprises aplurality of pairs 26 of acoustic sources 28 and hydrophones 30.However, the seismic survey system 20 may be constructed in a variety ofconfigurations that may comprise pairs 26 or other arrangements ofhydrophones 30 and acoustic sources 28. Generally, the hydrophones 30and acoustic sources 28 are selected such that there are at least asmany hydrophone measurements as there are unknown notional sources. Thisenables computation of the notional source signatures. In the specificexample illustrated, the acoustic sources 28 may each comprise an airgun (or air gun cluster) designed to emit pressure pulses at controlledpoints in time. The hydrophones 30 may be nearfield hydrophones that areeach positioned above a corresponding acoustic source or at othersuitable locations. The acoustic sources 28 and hydrophones 30 may bearranged in pairs 26 or other configurations that are, for example,suspended from a float 32 via appropriate suspension lines 34. Float 32is designed to float along a surface 36 of the water in the marineseismic survey area 24.

A control system 40, such as a computer-based control system, may beused to process data from hydrophones 30 and/or to convey controlsignals to the acoustic sources 28. The control signals can be used tocontrol the triggering of acoustic sources 28 to provide acoustic pulsesused in conducting a seismic survey. Data flow between control system 40and pairs 26 can be conducted over suitable communication lines 42. Byway of example, control system 40 can be positioned on a suitable towingvessel or at other locations, such as directly on the acoustic sourcesubarray 22.

The acoustic sources 28 and hydrophones 30 may be arranged in a varietyof array configurations. In the embodiment illustrated in FIG. 2, forexample, seismic survey system 20 comprises a seismic array 44 having aplurality of acoustic source subarrays 22. Each acoustic source subarray22 comprises a plurality of pairs 26 of acoustic sources 28 andnearfield hydrophones 30. By way of specific example, seismic array 44may comprise three subarrays 22 with each subarray comprising six pairs26. However, other configurations can be used to conduct seismicsurveys.

The reflected acoustic signals detected by hydrophones 30 and the dataprocessed by control system 40 are improved by optimizing acousticsource array performance, e.g., synchronization. According to oneembodiment, the acoustic signals are time-aligned directly. By way ofexample, acoustic source synchronization can be achieved usingband-limited notional source signatures. The general notional sourcetheory is described in various publications, such as U.S. Pat. No.4,476,553 (Ziolkowski et al.).

In FIG. 3, one example of seismic source array 44 is illustratedschematically in a plan view showing 18 unique pairs 26. Each individualhydrophone 30 of a given pair 26 measures the overall acoustic signalwhich has contributions from the corresponding acoustic source 28 aswell as the other acoustic sources 28 in array 44. In the graphillustrated in FIG. 4, for example, a pressure signal recorded by anindividual nearfield hydrophone 30 is illustrated by a solid line 46. Inthis sample, the pressure signal 46 is detected by the hydrophone 30located at position 17 of FIG. 3. The recorded pressure signal 46results from contributions of the acoustic sources and comprises asignal 48 from the closest acoustic source 28, e.g., the correspondingacoustic source 28 in the subject pair 26. The recorded pressure signal46 also comprises contributing signals 50 from the other acousticsources 28 in acoustic source array 44.

Even though the primary pressure pulses of individual acoustic sourcescannot be distinguished by individual hydrophones, implementation of thenotional source method enables calculation of the acoustic signals fromthe individual acoustic sources 28. The individual contributions are thenotional source signatures. Accordingly, control system 40 may comprisea processing system, e.g., a computer-based system, used to calculatethe acoustic signals from individual acoustic sources based on thenotional source method. The control system 40 is further used to processthe data and to determine triggering delays with respect to acousticsources 28 by time-aligning the notional source signatures. It should benoted that in many applications, the notional source signatures can berange or band limited to a frequency range of interest. As a result, theacoustic sources can be synchronized directly rather than relying solelyon indirect synchronization from acoustic source mounted time-breaksensors.

However, another embodiment of the methodology utilizes asynchronization method that combines the use of notional sourcesignature attributes and attributes associated with time-break sensorsignals. In this embodiment, pairs 26 can incorporate time-break sensorsmounted to, for example, acoustic sources 28. As described above, thetime-break sensors are used to detect an attribute associated with atime-break sensor signal. The attribute is used to estimate a time delaywith respect to triggering individual acoustic sources to optimize thecollection of data during a seismic survey. In this embodiment, controlsystem 40 is used to compare processed time delays based on detection ofthe attribute through time-break sensors with the direct synchronizationtime delays. The direct synchronization time delays are obtained throughthe calculation of notional source signatures and the determination oftriggering delays by time-aligning the notional source signatures. Thecomparison can provide validation or checks regarding the actualtriggering delays implemented.

In one example, the synchronization of an air gun array is optimized bycomputing an additional synchronization delay that relates a time-breaksensor to an emitted notional source signature, where the notionalsource signature is band limited to the frequency range of interest. Byway of example, the data can be band limited to approximately 0-128 Hz,however other desired seismic bands or ranges can be employed. Thesynchronization of notional source signatures also can be employed inother applications. For example, the approach can be used inapplications that utilize an emitted acoustic signal at frequenciesoutside the seismic band. The approach also can be used in applicationsthat do not necessarily aim to synchronize the primary pressure pulseemitted by the acoustic source elements of the acoustic array.

According to another embodiment of the methodology, the arraysynchronization may be optimized by desynchronizing the triggering ofthe acoustic sources 28. This allows the power spectrum of the seismicarray 44 to be attenuated at out of band frequencies, i.e., thefrequencies outside of a desired seismic frequency range/band.Additionally, the output within the desired seismic frequency band issubstantially unaffected.

Attenuation of the out of band signal is accomplished by deliberatelydesynchronizing the acoustic source triggering as controlled by controlsystem 40. When acoustic sources 28 are used to provide primary pressurepulses, the desired emission direction of the superpositioned wavefieldcan be described by:

${W(\omega)} = {\sum\limits_{n = 1}^{N}{{S_{n}(\omega)} \cdot {\exp \left( {- {j\omega\tau}_{n,{geo}}} \right)} \cdot {\exp \left( {- {j\omega\tau}_{n,{sync}}} \right)}}}$

Where S_(n)(ω) are the notional source signatures of the N sourceelements, ω is the angular frequency, τ_(n,geo) are the synchronizationdelays that compensate for the position of the source elements relativeto the desired farfield emission direction, and τ_(n,sync) are the timedelays that result from imperfect synchronisation due to the randomnature of the mechanical triggering together with sub-optimalperformance of the synchronisation control algorithm. It should be notedτ_(n,sync)=0 for the idealised case of perfect synchronisation.Furthermore, τ_(n,sync) generally have a non-zero mean when the seismicarray 44 is synchronised using time-break sensors, while a seismic arraysynchronised using the notional source signatures has τ_(n,sync) valueswith zero mean.

In the presently described embodiment, an additional time delay,τ_(n,scatter), is introduced to deliberately desynchronise the alignmentof the primary pressure pulses, such that:

${W(\omega)} = {\sum\limits_{n = 1}^{N}{{S_{n}(\omega)} \cdot {\exp \left( {- {j\omega\tau}_{n,{geo}}} \right)} \cdot {\exp \left( {- {j\omega\tau}_{n,{sync}}} \right)} \cdot {\exp \left( {- {j\omega\tau}_{n,{scatter}}} \right)}}}$

The additional time delays, τ_(n,scatter), are chosen so the out of(seismic) band amplitude is attenuated, while the amplitude within thedesired seismic band/range is substantially unaffected. Guidelines forthe choice of τ_(n,scatter) values can be found by analysing the powerspectrum of the normalised scattering function:

${W_{scatter}(\omega)} = {\frac{1}{N}{\sum\limits_{n = 1}^{N}{{\exp \left( {- {j\omega\tau}_{n,{scatter}}} \right)}.}}}$

Referring generally to FIG. 5, the power spectrum of this function isillustrated for the example when the time delays of the N=18 elementsare randomly drawn from a uniform distribution within [−L/2, +L/2] whereL is the length of the window to which the scattering delays areconfined. In many applications, L is greater than 1 millisecond. In theexample of FIG. 5, N=18, L=4 milliseconds, and the average out of bandattenuation is −12.6 dB for frequencies above about 236 Hz. When thescatter-delays are chosen from a random uniform distribution, theaverage attenuation of the out of band frequencies has been shown as:10·log₁₀). In this example, the attenuation is only −1.1 dB at 100 Hzfor the L=4 millisecond case. Furthermore, the lower end of the stopbandis approximately:

${f = \frac{N - 1}{N \cdot L}},$

where f is the linear frequency.

Additional examples of the power spectrum are illustrated graphically inFIGS. 6 and 7 for the cases when N=18; and L=8 milliseconds and 16milliseconds, respectively. In the examples of FIGS. 6 and 7, theaverage out of band attenuation is −12.6 dB for frequencies above about118 Hz and 59 Hz respectively.

Accordingly, the out of band amplitude of the signal can be attenuatedby deliberately scattering the acoustic source synchronization such thatthe time delays τ_(n,scatter) in the equation:

${W(\omega)} = {\sum\limits_{n = 1}^{N}{{S_{n}(\omega)} \cdot {\exp \left( {- {j\omega\tau}_{n,{geo}}} \right)} \cdot {\exp \left( {- {j\omega\tau}_{n,{sync}}} \right)} \cdot {\exp \left( {- {j\omega\tau}_{n,{scatter}}} \right)}}}$

have non-zero values. In many applications, these scattering delays areconfined to a window of length, L, of greater than 1 millisecond and thescattering delays, τ_(n,scatter), are chosen such that the combineddelay of the synchronization error, τ_(n,sync), and the scatteringdelays conform to a chosen statistical distribution. In the examples inFIG. 5-FIG. 8, the combined delay is randomly drawn from a uniformdistribution.

In some embodiments, the deliberate desynchronization is achieved byscattering the triggering times of the acoustic sources 28, however thedeliberate desynchronization also can be achieved by scattering theposition of the acoustic source elements. In the latter example, theacoustic sources are scattered such that the resulting perturbation tothe propagation time in the desired emission direction equals theprescribed scattering time delays. One example of the power spectrumarising from the deliberate desynchronization through scattering offiring/triggering times of the acoustic sources is illustrated in FIG.8. In this example, the deliberate scattering is again within a 4millisecond window and the average out of band attenuation is −12.6 dBfor frequencies above about 236 Hz. However, other power spectrums, suchas the generic power spectrums described above, may be used in otherapplications.

The systems and methodology for optimizing acoustic sourceperformance/synchronization can vary from one seismic survey applicationto another. However, one operational example is illustrated by theflowchart of FIG. 9. In this example, an array of acoustic sources andhydrophones is initially deployed in a marine seismic survey area toenable performance of a seismic survey, as represented by block 52. Theacoustic sources are used to generate acoustic pulses, as illustrated byblock 54. By way of example, the acoustic sources may comprise air gunsarranged individually or in clusters at specific array locations.

The notional source signatures are determined via, for example, controlsystem 40 based on data from nearfield hydrophone measurements, asrepresented by block 56. The notional source signatures are used toestablish triggering delays by time-aligning the notional sourcesignatures, as indicated by block 58. By establishing appropriatetriggering delays, the collection of seismic data during the survey iseffectively optimized. In some applications, optional time-break sensorsalso are utilized to detect an attribute associated with the time-breaksensor signal, as represented by block 60. Detection of the attribute bythe time-break sensors enables determination of related time delays thatcan be compared to the time delays that result from processing thenotional source signatures, as represented by block 62. The processingof data and the comparison of time delays can be conducted oncomputer-based control system 40.

In another operational example, acoustic sources 28 are initiallydeployed, as indicated by block 64 of the flowchart illustrated in FIG.10. Once the array is deployed, the acoustic sources are used togenerate acoustic pulses, as indicated by block 66. A desired frequencyrange or band of interest for seismic exploration also is determined, asrepresented by block 68. The desired frequency band is used to enableattenuation of out of band signals by deliberately desynchronizing theacoustic source triggering, as indicated by block 70. The computer-basedcontrol system 40 can be used to establish the parameters fordesynchronizing the acoustic source triggering and to initiate thedeliberate scattering of source triggering.

The examples discussed above are just a few of the configurations andprocedures that can be used to control acoustic source synchronizationin a manner that optimizes data collection during a seismic survey. Forexample, the number and arrangement of acoustic sources as well as thenumber and arrangement of hydrophones can vary from one application toanother. Similarly, the type of control system and the location of thecontrol system can be adapted to specific equipment and/or applications.Furthermore, the determination and use of notional source signatures canbe established according to various paradigms depending on, for example,system factors, environmental factors, sensor types, and other factors.Additionally, the desired frequency ranges/bands can vary betweenseismic exploration applications. Techniques for generating acousticinputs also may change depending on the environment and availableequipment.

Although only a few embodiments of the present invention have beendescribed in detail above, those of ordinary skill in the art willreadily appreciate that many modifications are possible withoutmaterially departing from the teachings of this invention. Accordingly,such modifications are intended to be included within the scope of thisinvention as defined in the claims.

1. A method, comprising: deploying an array of acoustic sources in amarine environment; creating acoustic pulses with the acoustic sourcesto obtain data on subsea structures; and deliberately scatteringacoustic source synchronization in a manner that attenuates theamplitude at out of band frequencies.